Six combinations of two incompatible polymers, polystyrene (PS) and poly (methyl methacrylate) (PMMA) of various molecular weights were made in toluene. This system separates into two phases upon approach to equilibrium. Compositions of the conjugate phases were determined using Gel Permeation Chromatography (GPC). The composite GPC Chromatogram of PS/PMMA mixture in the conjugate phases were successfully resolved into its component chromatograms which show the distribution of the molecular species in these phases. Application of the above GPC method to equilibrated samples yields tie lines, binodal curves and plait points of the ternary system.